1. Field of the Invention
The present invention relates to optical communication apparatuses, and particularly to an optical communication apparatus which modulates an electrical signal to an optical signal and transmits the optical signal.
2. Description of the Related Art
Current multimedia networks require optical communication apparatuses that can cover a very long distance and have a high capacity. Current optical communication apparatuses use an optical modulator as an electrical-optical converter.
FIG. 12 shows the configuration of an optical communication system. As shown in the figure, the optical communication system can be divided into a wavelength division multiplexing (WDM) transmission block for sending an optical signal, a transmission path for transmitting the optical signal, and a WDM reception block for receiving the optical signal. The WDM transmission block and the WDM reception block are configured by separate optical communication apparatuses.
Optical signals S1a, S1b, . . . , and S1n input to the WDM transmission block use a standard optical interface, which is the Synchronous Digital Hierarchy (SDH) or the Synchronous Optical Network (SONET). Optical-electrical converters (O/Es) 101a, 101b, . . . , and 101n convert the optical signals S1a, S1b, . . . , and S1n to electrical signals and supply the signals to optical modulators 103a, 103b, . . . , and 103n. 
The optical modulators 103a, 103b, . . . , and 103n receive light with different wavelengths from light sources 102a, 102b, . . . , and 102n. The optical modulators 103a, 103b, . . . , and 103n modulate the light output from the light sources 102a, 102b, . . . , and 102n in accordance with the electrical signals output from the O/Es 101a, 101b, . . . , and 101n, and output optical signals with wavelengths of λ1, λ2, . . . , and λn to an optical multiplexer 104. The optical multiplexer 104 wavelength-multiplexes the optical signals with wavelengths of λ1, λ2, . . . , and λn and outputs the result to the transmission path.
The transmission path includes, for instance, optical fibers 111 and 113 and an optical amplifier 112, as shown in the figure. The optical amplifier 112 amplifies the optical signal attenuated in the transmission path.
An optical demultiplexer 121 of the WDM reception block divides the optical signal transmitted through the transmission path to optical signals with wavelengths of λ1, λ2, . . . , and λn. Photodetectors 122a, 122b, . . . , and 122n convert the divided optical signals with wavelengths of λ1, λ2, . . . , and λn to electrical signals and output the signals to electrical-optical converters (E/Os) 123a, 123b, . . . , and 123n. The E/Os 123a, 123b, . . . , and 123n convert the electrical signals from the photodetectors 122a, 122b, . . . , and 122n to optical signals S2a, S2b, . . . , and S2n of a standard optical interface, SDH or SONET, and output the signals to a subsequent circuit. Signals are sent and received as described here.
The shown optical modulator 103a will be described in further detail. FIG. 13 is a block diagram showing the configuration of a conventional optical modulator. FIG. 13 shows the configuration of the optical modulator 103a shown in FIG. 12. The figure also shows the light source 102a shown in FIG. 12. The optical modulators 103b to 103n shown in FIG. 12 have the same configuration as the optical modulator 103a, and a description of those optical modulators will be omitted.
The optical modulator 103a includes an optical intensity modulator 131, which is an interference-type optical modulation element, a driver 132, an optical tap 133, a photodiode (PD) 134, and an automatic bias control (ABC) circuit 135.
The driver 132 receives data and a clock. The data input to the driver 132 is an electrical signal output from the O/E 101a, described with reference to FIG. 12. When a 10-Gb/s clock is input, for instance, the driver 132 outputs 10-Gb/s data to the optical intensity modulator 131.
A modulating electrode 131a of the optical intensity modulator 131 intensity-modulates the light output from the light source 102a in accordance with the data output from the driver 132. The optical intensity modulator 131 outputs the modulated data (optical signal) to the optical multiplexer 104, which was described with reference to FIG. 12, and a part of the data is branched by the optical tap 133 to the PD 134. The PD 134 detects the optical level of the optical signal and outputs it as an electrical signal to the ABC circuit 135.
When an interference-type optical modulation element is used to perform optical modulation, a direct-current (DC) voltage (bias voltage) is applied to the modulation signal in order to fix the operating point of electrical-optical conversion to a certain position. In FIG. 13, a bias voltage is applied to an electrode 131b of the optical intensity modulator 131. The operating point of the interference-type optical modulation element, however, varies with time or varies with the applied voltage or the like, and a drift is generated. Accordingly, the ABC circuit 135 performs feedback control so that an optimum bias voltage is applied to the electrode 131b. 
FIG. 14 is a block diagram showing the configuration of the ABC circuit 135. A DC bias generator 141 of the ABC circuit 135 outputs a digital bias voltage that determines the operating point of the optical modulation signal. A digital-to-analog converter (DAC) 142 converts the bias voltage to an analog signal, and a non-inverting amplifier 143 amplifies the voltage. The non-inverting amplifier 143 outputs the analog bias voltage to an adder 146.
A pilot signal generator 144 generates a pilot signal for detecting a shift of the operating point because of the bias voltage. A digital-to-analog converter (DAC) 145 converts the pilot signal to an analog signal. The adder 146 adds the converted analog pilot signal and the bias voltage, and an inverter 147 inverts the output. The inverter 147 outputs the inversion of the sum of the pilot signal and the bias voltage to the electrode 131b shown in FIG. 13.
The optical signal containing the pilot signal, optically modulated by the optical intensity modulator 131 shown in FIG. 13, goes through a waveguide of the optical intensity modulator 131, and the optical tap 133 branches a part of the signal to the PD 134. The PD 134 detects the optical level of the optical signal and outputs a corresponding electrical signal to a transfer impedance amplifier (TIA) 148. The TIA 148 extracts an alternating-current component of the electrical signal and performs current-voltage conversion. The electrical signal converted by the TIA 148 is filtered by a band pass filter (BPF) 149 having a center frequency of 1 kHz, that is, the pilot signal is extracted from the electrical signal. An amplifier (AMP) 150 amplifies the pilot signal in accordance with the input range of an analog-to-digital converter (ADC) 151, and the signal is converted to a digital signal.
The pilot signal generator 144 outputs the pilot signal also to a phase shifter 153. The phase shifter 153 adjusts the phase of the pilot signal output from the pilot signal generator 144, in consideration of a change in phase of the pilot signal output to a multiplier 152 through the optical intensity modulator 131 and the like. The multiplier 152 multiplies the digital pilot signal converted by the ADC 151 by a reference signal output from the phase shifter 153 and outputs the result to a low pass filter (LPF) 154. The LPF 154 extracts a direct-current component from the pilot signal multiplied by the multiplier 152 and outputs the component to a DC bias optimal control circuit 155.
The multiplier 152 and the LPF 154 perform synchronous detection (lock-in amplification) of the pilot signal. While the bias voltage is at the optimum operating point, 0 V is obtained as a result of synchronous detection. If the result is not 0 V, the DC bias optimal control circuit 155 changes the bias voltage of the DC bias generator 141 accordingly, so that the optimum operating point can be maintained. Through the operation as described here, the optimum value of the bias voltage is maintained.
FIGS. 15A to 15C illustrate the synchronous detection of the pilot signal. The pilot signal output from the ADC 151 is shown in FIG. 15A. The reference signal output from the phase shifter 153 is shown in FIG. 15B. The output of the multiplier 152 is shown in FIG. 15C.
FIG. 15C shows the result obtained by multiplying the pilot signal shown in FIG. 15A by the reference signal shown in FIG. 15B. When the LPF 154 passes the electrical signal shown in FIG. 15C, the pilot signal can be detected as a direct-current value.
One proposed optical modulator suppresses a shift of the operating point because of a variation in the power of light output from the modulator while specifying a desired operating point of an external modulator (refer to Japanese Unexamined Patent Application Publication No. 2000-171766, for instance). In contrast to a nest-type optical intensity modulator using a combination of three Mach-Zehnder (MZ) waveguides, one method is proposed to control the bias voltage of an optical modulator appropriately by a simple configuration, and one apparatus using the method is also proposed (refer to Japanese Unexamined Patent Application Publication No. 2004-318052, for instance).
If noise is large for the optically-modulated pilot signal, the signal output from a synchronous detection circuit (the multiplier 152 and the LPF 154, in the above case) contains an offset caused by the noise, and it is hard to control the operating point of the optical modulator appropriately.